I've shot basketball with the D300 with flash. You need to be at least two stops below the correct non-flash exposure to freeze the action with the flash. I would set my camera to manual and set the shutter at 1/250 with the lens wide open. I would then set the ISO at the setting that underexposed by two stops. For me this was 800.
There is some discussion about using the flash in manual or in TTL. I preferred to use TTL as the players weren't always the same distance from the camera. I also had to play with the flash compensation to keep the hot spots down. If you can find a surface to bounce the flash off of, so muc the better.